1. Field of the Invention
The present invention relates to a process for the production of organic compounds from a synthesis gas containing hydrogen and carbon oxides. The organic compounds prepared by this process may be a hydrocarbon, a mixture of hydrocarbons, an oxygenated compound (such as an alcohol, an ether, an ester, an acid, an anhydride, a ketone), or any mixture thereof. The invention is particularly useful for the production of methanol.
2. Description of the Prior Art
Processes for the preparation of various organic compounds from a synthesis gas containing hydrogen and carbon oxides are described in many prior patents and publications. By way of example: U.S. Pat. Nos. 2,829,313, 3,962,300, and 4,464,483, describe processes of the production of methanol; U.S. Pat. Nos. 4,413,064 and 4,088,671 describe processes of the production of hydrocarbons; M. J. Van der Burgt and S. T. Sie in a paper presented at the PETRO PACIFIC symposium at Melbourne, Australia, 16-19 September 1984 describe processes for the production of liquid hydrocarbons; and Ph. Courty et al "C.sub.1 -C.sub.6 Alcohols from syngas" in Hydrocarbon Processing at page 105 (November 1984) describe the production of alcohols.
In these prior art processes, the organic compound is formed in a closed synthesis loop which includes the reactor in which the compound is formed and associated heat exchangers which permit separation of the desired product and recycle of the unreacted gases. Fresh synthesis gas is injected into the loop where it is combined with the recirculating gases. The mixture of fresh synthesis gas and recirculating gases are then fed to the reactor. The effluent from reactor containing the desired organic product is introduced into a heat exchanger wherein it is cooled to a sufficiently low temperature to cause the organic product to condense. The condensed product is withdrawn from the loop. Gases that are not condensed are recycled back to the reactor. However, a portion of these recycled gases is continuously purged from the loop in order to maintain the concentration of inerts, such as methane, argon, and nitrogen, at a reasonable level.
Although the present invention can be used for the production of numerous organic compounds, the remainder of this specification will focus on methanol since it is a large tonnage industrial product. Methanol is synthesized commercially by reforming a synthesis gas containing hydrogen, carbon monoxide, carbon dioxide, and small amounts of inert gases such as methane and nitrogen. The carbon oxides react with hydrogen to form methanol according to the following equations: EQU CO+2H.sub.2 .fwdarw.CH.sub.3 OH EQU CO.sub.2 +3H.sub.2 .fwdarw.CH.sub.3 OH+H.sub.2 O
The synthesis gas is conveniently characterized by the following ratio of hydrogen to carbon oxides: ##EQU1## This synthesis gas composition is stoichiometric when Z=1.00. In the production of higher alcohols from synthesis gas, the optimum synthesis gas composition is also very close to Z=1.00, as reflected by the following general equations: EQU nCO+(2n)H.sub.2 .fwdarw.CH.sub.3 (CH.sub.2).sub.(n-1) OH+(n-1)H.sub.2 O EQU nCO.sub.2 +(3n)H.sub.2 .fwdarw.CH.sub.3 (CH.sub.2).sub.(n-1) OH+(2n-l)H.sub.2 O
The Z ratio of hydrogen to carbon oxides is not universally used. By way of example, in U.S. Pat. No. 4,413,064, CO.sub.2 is not considered as an active component in the reaction and the hydrogen to carbon oxides mole ratio is described in terms of H.sub.2 /CO. The preferred H.sub.2 /CO mole ratio described in that patent is between 1.5 and 2.0. If CO.sub.2 is included in the ratio, the optimum synthesis gas composition would actually correspond to a Z ratio appreciably lower than 1.00. Thus, when comparing the ratios of hydrogen to carbon oxides used in different processes, it is necessary to determine which carbon oxides are included in the ratio.
The optimum synthesis gas composition is the one that permits the use of the lowest pressure in the methanol synthesis loop for a given production rate, everything else being equal. This optimum composition may be identical to the stoichiometric composition (where Z=1.00). However, because (1) of kinetic reasons connected to the activity and selectivity of the synthesis catalyst and (2) of differences in solubilities of the various reacting gases in liquid methanol, the optimum ratio may be slightly different from stoichiometric.
In the conventional process for producing methanol from a light hydrocarbon feedstock, ranging from natural gas to naphtha, a desulfurized feedstock is steam reformed at moderate pressure, in the range of 15 to 25 atm, and a high temperature, in the range of 850.degree. to 900.degree. C. The reforming reaction is endothermic and occurs in a reactor comprising refractory tubes externally heated by a set of burners, and filled with a fixed bed of catalyst made essentially of nickel on a refractory support. The synthesis gas is then cooled and compressed to the pressure used for the methanol synthesis, which ranges from 50 to 100 atm in the so-called "low pressure" processes, and which may reach 300 atm in the older high pressure processes. The pressurized gas is then introduced into the synthesis loop.
Because of the low carbon/hydrogen ratio of the light hydrocarbon feedstocks and the minimum steam rate which must be used in steam reforming, the synthesis gas produced has a composition very different from the stoichiometric composition required for methanol syhthesis. As a result, the synthesis loop operates with a very large excess of hydrogen. In addition to the non-stoichiometric synthesis gas composition, these prior art processes have a number of disadvantages which are particularly significant if a large capacity plant, i.e., one producing in excess of 2000 metric tons/day, is being used.
Because of the presence of excess hydrogen, the rate at which gases are purged from the loop must be very high. This results in a loop capacity that is appreciably lower than could be achieved if the synthesis gas had the stoichiometric composition. Furthermore, the reforming of the feedstock occurs at a low pressure. When the low-pressure is coupled with the high purge rate from the synthesis loop, it results in a poor overall efficiency.
Another disadvantage of these prior art processes is the excess CO.sub.2 in the synthesis gas. Since the synthesis gas contains excess hydrogen and carbon dioxide, larger amounts of gases must be pressurized than would be necessary if the composition was stoichiometric. Because of the large quantity of synthesis gas that must be compressed, the horsepower and the dimensions of the synthesis gas compressor become excessive for methanol capacities above 2000 tons/day. The high CO.sub.2 content creates another problem. It results in the formation of significant amounts of water in the synthesis loop, thereby increasing the cost of fractioning the methanol-water mixture that is condensed in the synthesis loop.
Finally, the cost of the steam reforming heater, which is a very large fraction of the overall plant cost, increases approximately linearly with capacity. This means that very little gain can be achieved by scaling up to a large single train capacity.
In place of the above-described conventional steam reforming process, a so-called "combination process" could be used. In this process the whole feedstock undergoes first a primary steam reforming reaction and then a secondary reforming with oxygen, in a single stage reactor operating adiabatically and packed with a single catalyst bed. Such a process, as described in U.S. Pat. No. 3,388,074, is widely used in the ammonia industry in which air is replaced by oxygen. Although this combination process allows the use of higher operating pressures in the synthesis gas generation, it does not easily achieve a final synthesis gas having the optimum composition required for methanol synthesis due to the minimum amount of steam that must be used in the primary steam reforming reaction. For the same reason, it does not permit the formation of a synthesis gas having a low CO.sub.2 content. Furthermore, the large size of the primary steam reformer requires a high investment cost.
In U.S. Pat. No. 3,278,452, a process is described for the production of hydrogen and synthesis gases, in which part of the feedstock undergoes a primary steam reforming reaction, the effluent is mixed with the other fraction of the feedstock, and the mixture obtained is passed, in a secondary reforming reactor, through a succession of conversion zones with oxygen introduced between each until the desired conversion is reached. While this process, which is essentially oriented toward the production of hydrogen and ammonia synthesis gas, may to some extent yield a gas approaching the stoichiometric composition required for methanol synthesis, it still leads to a high CO.sub.2 content in the synthesis gas and it requires a costly multistage reactor to perform the oxygen reforming reaction. Furthermore, the injection of oxygen between the successive catalyst beds, operating at very high temperatures, requires the solution of very elaborate technological problems. A multistage oxygen reforming reactor is required in this process because of the high concentration of hydrocarbons in the feed to the secondary reformer. If all the oxygen was introduced in a single stage reaction, carbon formation wold result and excessive temperatures would be required in the secondary reformer.
Furthermore, it has been reported in the prior art, as outlined in U.S. Pat. No. 3,278,452, that in a single stage secondary oxygen reforming of a hydrocarboncontaining feedstock, the maximum amount of conversion that may be achieved is such that the percentage methane equivalent of the product gas is about one-fifth of that of the feedstock, when the latter is above 25 percent. The expression "percent methane equivalent" as used herein means mole percent of hydrocarbons expressed as methane on a dry basis, e.g., ten mole percent ethane is 20 percent methane equivalent.
In British Patent No. 1,569,014, a process is described for the production of a synthesis gas having essentially the stoichiometric composition for methanol synthesis, that is with a Z ratio very close to or equal 1.00. In this process, a fraction of the feedstock is steam reformed in a primary steam reformer after which it is combined with another fraction of the feedstock, and the mixture is reacted with oxygen in a single stage secondary reformer operating under essentially adiabatic conditions. This process has the advantage of avoiding all the drawbacks mentioned above for the conventional processes, and in particular of reducing the investment cost, mostly due to an appreciable reduction of the size of the steam reforming heater. However, in order to achieve a near stoichiometric composition on the final synthesis gas, there is a limitation to such a reduction of the steam reformer size in the process. This is so because, in the primary steam reformer, the amount of hydrogen produced is much more than the soichiometric amount corresponding to the carbon oxides produced, whereas in the secondary oxygen reformer more hydrogen is burned than carbon monoxide. In other words, the primary steam reformer leads to a high Z ratio, whereas the secondary oxygen reformer reduces the Z ratio of the synthesis gas. Therefore, a balance should be maintained between the primary and secondary reformers in order to reach a stoichiometric composition at the outlet of the secondary reformer.
In the process of the present invention, which also combines a primary steam reformer with a secondary oxygen reformer, there is no need to maintain a balance between the primary and secondary reformers, because the synthesis gas composition at the outlet of the secondary reformer deviates purposely from the stoichiometric composition, with a Z ratio appreciably lower than 1.00, and, therefore, much less reforming is performed in the primary reformer, which reduces appreciably the cost of the overall plant.